Closes https://github.com/MusicPlayerDaemon/MPD/issues/1001

To be reused by the FFmpeg filter plugin.

Fixes regression by commit 23d5a2b8 -
that commit always pretended that any Opus file has both track and
album gain, and thus disabled the fallback to the other if one is not
set.

This patch changes the logic to only submit ReplayGain if at least one
value is set, and apply the offset only to that value.  If none is
available, then the new check in HandleAudio() will submit only the
output gain.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/977

Found with modernize-use-auto
Signed-off-by: Rosen Penev <rosenp@gmail.com>

"DEFAULT" is a bad name - all it says is that it's the default value,
but it doesn't say what it means.  The name NOTICE mimics the syslog
level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/892

This implements the feature that was missing/broken in this bug
report: https://github.com/MusicPlayerDaemon/MPD/issues/930

Prepare for an implementation without InputStream.

Similar to commit 4e2a551f but for
decoder plugins.  This is tailored for the FFmpeg decoder plugin which
implements some protocols (e.g. RTSP) as demuxer plugin.

Support opus header gain tags and match opus playback volume to other tracks when ReplayGain is enabled.

Found with -Wdouble-promotion
Signed-off-by: Rosen Penev <rosenp@gmail.com>

While libsndfile doesn't like partial reads in the middle of a file
(see commit 95ac6071), it allows partial reads at the end of a file.
It doesn't pay attention to the file size when issuing a read.

Commit ecb67a1e (MPD 0.18.12) was a regression: previously,
partial reads at the end of a file were possible, but switching to
decoder_read_full() made this an error condition.  This way, a portion
at the end of each file was lost, leading to corruption with gapless
playback (https://github.com/MusicPlayerDaemon/MPD/issues/936).

This fix switches to the newly introduced function
decoder_read_much(), which does the same as the code before commit
ecb67a1e.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/936

Prepare for templated functions.

Bug #915 is about an I/O exception thrown where none was allowed,
leading to crash via std::terminate().  However, instead of catching
and logging the error inside the decoder plugin, it should be able to
propagate the I/O error to the MPD core, so MPD can avoid trying other
decoder plugins.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/915

Closes https://github.com/MusicPlayerDaemon/MPD/issues/867

Will be needed for End Trimming (RFC7845 4.4,
https://github.com/MusicPlayerDaemon/MPD/issues/867).

Fixes memory leak.  That's what we get for

Fixes the first part of
https://github.com/MusicPlayerDaemon/MPD/issues/867

When the client wants to seek, but the decoder has already finished
decoding the current song, the player restarts the decoder with an
initial seek at the new position.  When this initial seek fails, MPD
pretends nothing has happened and plays this song from the start.

With this new flag, a restarted decoder marks the initial seek as
"essential" and fails the decoder if that seek fails.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/895

Before the advent of io_uring (commit dae8da70), this didn't
matter, because the `FileInputStream` never called this.  But
`UringInputStream` is derived from `AsyncInputStream`, and needs the
handler to signal completion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/898

The ones in std have overloads for const char/char.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Fixes regression from commit 69f09648